The production of reinforced metal matrix composites ("MMC") which result in parts with exceptional strength, resistance to wear and to heat, has, to date, only been possible with relatively simple shapes at a cost too high to be practical for any but the most demanding applications. A typical MMC is produced by squeeze-casting a porous ceramic or porous metal core having a higher melting point than the metal part to be reinforced. A porous core in combination with "plugs" for bores, and surface spacers to provide a skin of desired thickness, as will be explained herebelow, is referred to as a "preform" though it may not have a shape which closely conforms to that of the squeeze-cast article to be produced. A metal reticulate of titanium (say) may be impregnated with aluminum, but the invention will be more particularly directed to an open pore ceramic reticulate because ceramic is more commonly used. In a large squeeze-cast article, more than one insert may be used, but more typically, only one is used, and the invention is described using only one integral open pore ceramic insert.
The integral open pore ceramic "preform" is typically placed in a squeeze-casting press; enough molten metal is poured over the preform to impregnate it, and the press is closed. Sufficient pressure is then exerted to impregnate the ceramic preform with the metal.
The preferred "preform" used in my invention is so shaped that, when covered with a metal skin, it corresponds to the shape of the finished, squeeze-cast article. Such an article has a "net" or "near-net" shape requiring very little, if any, machining to meet finished tolerances, and, more preferably, essentially no machining.
The preform may have a shape which does not conform approximately to that of the finished squeeze-cast article. A typical example is that of a squeeze-cast metal part reinforced with a deliberately positioned, bundle of continuous fibers having a higher melting point than the metal, and high tensile strength, positioned so as to provide directional reinforcement where desired; or, a metal part formed by impregnating a shaped mass of metal fibers to provide more general reinforcement. In either case, such a preform is referred to as being a "fibrous insert".
A preform may be a ceramic or metal reticulate in which there is open communication between substantially all pores, but not have approximately the same shape as the desired finished part; or, the preform may be an elongated metal reticulate of arbitrary cross section optionally, additionally, directionally reinforced with an arbitrarily shaped sheet of metal, each having a higher melting point than that of the molten metal used to form the part. In any of the foregoing cases, such a preform is referred to as a "non-fibrous insert".
The production of a metal-impregnated reinforced body, in which a ceramic preform is squeeze-cast to form the MMC, is limited by the present state of the art, to a simple geometric shape, such as a dome or cylinder. It is impractical, from an, economic point of view, to squeeze-cast an object even of such simple shape because it takes so long for the molten metal to solidify, and pressure is maintained during the entire period - unless, of course, if the price of the finished article is inconsequential. In addition, excess metal must typically be removed from the squeeze-cast article. The known process is uneconomic, irrespective of which metal is used, whether essentially unalloyed, or not, and is particularly true for aluminum, magnesium and steel.
The incentive to make MMCs of light metal alloys is particularly great because metals such as aluminum and magnesium have relatively poor resistance to high temperature, to fatigue, and to wear by friction, combined with a relatively low modulus of elasticity compared to steel (say). All of which properties are greatly improved by a molten metal-impregnatable shaped fibrous or non-fibrous insert, preferably a ceramic porous body, shaped so that upon being covered with a metal skin, there results a metal MMC in a "net shape" or "near-net shape". A fibrous insert, for example, one made with reinforcing inorganic, metallic or non-metallic, particularly ceramic, fibers may be appropriately shaped to provide directional reinforcement in a preform. A preform may also be made by combining an integral ceramic preform with fibers.
Of course, the above-identified deficiencies of light metals and their alloys can be negated by randomly distributing short fibers throughout the melt, before casting the part. However, mixing the fibers in the melt before it is squeeze-cast, referred to as being "compocast", produces parts which are far from being comparable in performance to parts squeeze-cast with a fibrous insert. Compocast parts are notably inferior compared with either an insert of an assembly of relatively long fibers more than about 1 cm long, or, a non-fibrous insert of a relatively large single-piece (integral), or, multiple-piece porous ceramic reticulate.
An assembly of fibers may be bundled to provide a shape which is close enough to the desired finished part to provide a near-net shape; or, a bundle of fibers may be overlaid and held in place on a shaped non-fibrous insert having a shape which closely conforms to the desired finished part to yield a "net" or "near-net" shape. Whether an open pore ceramic reticulate, a metal sheet, or a bundle of ceramic or metal fibers, or a combination of two or more thereof, such preforms are preferably so shaped that, upon melt-impregnation and being covered with a metal skin, they are recovered in a "net shape" of the MMC to be manufactured, or a "near-net" shape thereof. But designing a mold to impregnate either a bundle of fibers or a ceramic preform, is a complicated problem. It is a more complicated problem to design a mold to squeeze-cast and thoroughly impregnate a bundle of fibers disposed on a ceramic preform (the combination is sometimes termed a "hybrid preform" but is referred to in the illustrative example provided later herein, simply as a "preform").
A further complication ensues if the MMC is to be squeeze-cast in a ceramic mold rather than a metal mold (because the molten metal is at too high a temperature for an affordable metal mold). A ceramic mold cannot withstand the several thousand pounds per square inch (Ksi) pressure generated during squeeze-casting unless it is perfectly matched to the shape of the metal mold cavity in which it is placed. This mandates squeeze-casting only simple geometric shapes in a ceramic mold perfectly fitted in a metal die.
Even where metal squeeze-casting molds are used, only simple shapes can be formed. For example, in a typical squeeze-casting process for a cone-shaped MMC, the tooling includes a closely tooled punch which is insertable in a downwardly tapered mold (sometimes referred to as a die) lined with a shaped mass of ceramic or steel fibers shaped as a mat conforming to the shape of the cone desired, and to the inner surface of the mold (the walls of the mold cavity). The tooling is lubricated and preheated before the molten charge of metal (say aluminum) is poured into the mold cavity lined with the fibrous mat. While the melt is liquid, the punch is lowered into the mold cavity, tightly closing it, and the punch exerts sufficient pressure to force the melt into the pores of the mat preform. The closed position of the tooling is maintained until the melt solidifies under pressure. Then the squeeze-cast part is ejected, for example by a ram which moves upward against the outside bottom surface of the MMC. This conventional process is more fully described and illustrated herebelow.
It immediately will be evident that sophisticated engineering and close-tolerance tooling is required to squeezecast in the range above about 66.7 Mpa (10 Ksi), even when the part is a simple shape. It will be equally evident that (i) a complex shape cannot be formed in this manner; and, (ii) the time required to cool the tooling for even a part having relatively small dimensions, becomes an onerous economic consideration. Clearly, removing the pressure on the squeeze-cast part in the die was never considered, because there are no provisions for removing the squeeze-cast metal shape while the metal impregnating the preform while the metal is still molten.
There has been no suggestion in the prior art that the time required to squeeze-cast a MMC, then cool it, should be severable, that is, split into two or more time periods.
We have found that in a great number of squeeze-casting operations removing the pressure from the still-molten metal does not adversely affect the strength of the MMC formed, and in such instances, this invention affords an elegant solution to the problems of molding a MMC of complex shape. At the same time, the process of this invention divorces the time required to squeeze-cast metal into the preform, from the time required to cool the metal to solidify it. I have effected such a divorce of essential time periods by combining portions of techniques used in investment casting, in die casting, and in squeeze-casting a MMC.
The logical choice for casting complex shapes is investment casting. As is well known, in investment casting, a wax is injected into a pattern die; the wax pattern is removed from the die; where a relatively small part is to be manufactured, for example, the receiver for a handgun or rifle, several patterns are assembled to wax runners to form a "tree"; the tree is dipped or invested (in a slip of ceramic particles); additional layers, starting with fine sand or other ceramic particles, are applied to the tree in a stucco process; then the stucco shell is dried and dewaxed.
Since the shell is to be used for the mold in which the MMC is to be formed in my process, the mold is referred to as a shell-mold. The shell-mold is hereafter referred to as a "mold" for brevity, and is referred to as a "shell-mold" to distinguish the ceramic shell-mold from a metal "die" (so referred to, instead of referring to it as a "mold", to avoid confusion) in which the shell-mold is to be cradled.
In a conventional investment casting process, the dewaxed mold is preheated; the molten metal is then poured into the hot mold; and the mold is broken away from the casting after it is cooled. The individual parts, which are dimensionally essentially identical to the patterns, are cut from the runners which connect the parts to the ,trunk, of the tree.
It must be remembered that, by definition, in investment casting, no preform, or insert of any kind, is left in the shell-mold. The basic concept of producing a shell-mold is tied to the only reason for doing so, namely, to produce a cavity of the desired shape which the molten metal is to assume. The concept of maintaining a preform within a shell-mold can only derive from the specific intention of using the combination of the preform and shell-mold for a particular purpose, and such a purpose would appear to rule out squeeze-casting as it is presently practiced. Further, using a shell mold in a squeeze-casting process requires that the shell-mold withstand very high hydrostatic pressure. One skilled in the art of casting knows that investment casting is not used in pressure casting situations, and would have no reason to consider using a shell-mold under high pressure conditions.
Still further, the choice of a shell-mold such as is typically used for investment casting, begs to be discarded as soon as it is considered, because, even if one could insert a punch through a passage (through which wax is removed) in the shell-mold, there is no known manner to cushion the outer surface of a frangible ceramic shell-mold in a metal die cavity in such a way that the shell-mold can withstand high pressure exerted by the punch. The slightest non-uniformity of the outer surfaces of either the die or the shell-mold, will cause the shell-mold to crack once the punch exerts much pressure upon molten metal poured into the shell-mold. There appeared to be no practical way to solve the problem. This invention provides a solution to that problem.